Thursday, May 31, 2007

Inadvertent chemical reactions

This past week I was cleaning some Tupperware containers that had had moldy food in them. I have discovered that one of the best way of cleaning stained/smelly plastics is household hydrogen peroxide and sunshine. I had put the containers on roof of my car in the sun and then filled them with hydrogen peroxide. After leaving them outside for an hour or so I brought them in.

On the way inside through the garage I spilled some of the hydrogen peroxide onto a stain on the garage floor. On the way back out to the car to get a few more of the containers I saw the garage floor was steaming where the stain and the spilled hydrogen peroxide coincided.

Being an engineer I was reflected on this and it made perfect sense. The stain was some hydrocarbon and the hydrogen peroxide a strong oxidizer (though diluted for household use) when they came in contact they reacted. fortunately not very energetically.

Here is someone else's experience with an unplanned experiment. Unfortunately his was a bit more energetic.

I have reproduced the account below:

A safety coordinator at the Esso Oil Company plant in Longford, Australia, was using a belt grinder in his home workshop to smooth the edge of a hacksaw cut on a 2" length of 1.5" angle iron. He had been grinding for about 1.5 to 2 minutes when there was a loud "THUMP" accompanied by an approximately 2-foot diameter brilliant yellow orange fireball. The fireball lasted no more than 1 second and then completely extinguished itself. It completely enveloped the machine, his hands to half way up his forearms, and the front of his torso.

Injuries included deep second-degree burns to about 60% of the victim's left hand and 50% of his right hand and first degree burns to his neck, chin, cheeks, lips, and the end of his nose. The right cuff of his shirt was smoldering, his face felt a burning sensation, and he could hear the front of his hair sizzling. Nothing on the bench was burning. A few streaks of white powder were deposited on the bench top and on a few items lying on the bench. The workshop was filled with dense white smoke with very little odor. His fingers and the ends of his thumbs escaped relatively unscathed as they were protected from the heat flash. He was wearing glasses, which protected his eyes. He also lost half his moustache, one of his eyebrows, and about 1 inch off the front of his hair. His eyelashes were curled by the heat but not singed. The burns to his face were caused solely by radiant heat, as the fireball did not come that high.

Analysis:
A few days before the event, the man's son had ground the heads off about twelve aluminum pop rivets. Finely divided aluminum mixed with finely divided ferrous oxide (the black powder residue from grinding steel) produced a compound called thermite. Thermite is used to fill incendiary bombs and is used commercially to weld large steel items. It burns at approximately 3500C (6300F), hence the extensive burns from such a short exposure time.


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chemistry
accident analysis
garage safety

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